This invention relates to apparatus for sensing RF current delivered to a plasma in a plasma chamber, for example a process reactor. The invention also relates to a waveform sampling circuit for use, inter alia, with such apparatus.
By accurately measuring the current-voltage characteristic of the power delivered to the plasma in a process reactor, the plasma can be monitored to give useful information on the plasma process (for example, etching or deposition) and allow better control of that process.
In order to characterise the plasma the current and voltage waveforms inside the chamber at the surfaces in contact with the plasma are required. Because of the intrinsic impedance of the chamber itself the current and voltage waveforms at the point outside the chamber where the sensor is located are different from those at the electrode surface or antenna. More importantly, the chamber impedance affects each frequency component of the current and voltage waveforms differently. However, the values of the chamber impedances (Z) can be established by the application of a harmonic rich signal with a fundamental of 13.56 MHz (or other normal operating frequency) to a chamber without creating a discharge. At each harmonic the equation Z(w)=V(w)/I(w) is obeyed, where V(w) is the complex voltage and I(w) is the complex current at the angular frequency w.
If the plasma chamber impedance network is represented by four unknown values, chamber resistance, electrode inductance, electrode-to-ground capacitance and stray capacitance, as shown in FIG. 1, then the values of the impedance of the chamber at each of the first five harmonics Z(w1), Z(w2), Z(w3), Z(w4) and Z(w5) can be used in a non-linear search algorithm to establish the most likely values of the impedance of the network in the absence of a plasma. Once the network impedance values are established then the current and voltage waveforms present at the electrode in the presence of a discharge can be obtained. Once these are known the true plasma impedance can be obtained. This technique allows the monitoring of the state of the plasma during normal operation but also requires a measure of the electrical state of the chamber in the absence of a discharge (the chamber network values). This provides a powerful plasma machine diagnostic as well as a diagnostic of the plasma within the machine.
In order to separate the current components flowing in the network from that flowing in the plasma and to obtain reliable values of the impedance of the chamber it is necessary to obtain very high resolution measurements of the current-voltage waveforms near the chamber. The relative phase between the current and voltage is also required to obtain the complex values of both waveforms.
In principle, voltage and current measurements are straightforward. In practice the main technical difficulties are due to inadequate shielding of the current and voltage sensors from stray fields. The most important aspect of pickup from stray fields is the effect on phase. For example, if the current is represented by I=I.sub.0 Sin(wt) and the voltage V=V.sub.0 Cos(wt), if the accuracy required in the measurement of the phase is to be less that 0.1 degrees then the cros-stalk between current and voltage channel when the measured current and voltage signals are approximately equal must be less than -60 dB. If the output of the voltage channel greatly exceeds the current signal then the cross-talk criterion is even more severe. This shift in phase due to cross-talk is very important particularly in the calculation of real power delivered to a plasma. Similar problems can arise if stray magnetic flux cuts the current pick-up coil from out side the sensor head which has a different phase from the current being measured. Other causes of phase shift can in principle be removed by careful calibration but the problem of stray external fields cannot.
The standard approach to sensing the current and voltage signals is shown schematically in FIG. 2 which shows a conductor 10 along which RF current flows to a plasma chamber (not shown). The conductor 10 is surrounded by a grounded metal shield (Faraday shield) 12, only one side of which is shown in FIG. 2. The current and voltage signals are measured using a single loop 14 for the current sensing and a capacitor 16 for the voltage sensing. Each sensor 14, 16 is connected in series with a respective 50 ohm resistor 18 through a respective conventional BNC connector 20. The voltages V.sub.i and V.sub.v developed at the resistors 18 are given by: EQU V.sub.i =MdI/dt and V.sub.v =RCdV/dt
where M is the mutual inductance between the loop 14 and the current path 10, C is the capacitance between the current path 10 and ground via the 50 Ohm resistor 18, and I and V are respectively the RF current and voltage flowing in the RF conductor 10.
Having obtained V.sub.i and V.sub.v, a waveform sampling circuit is used to extract the amplitude and phase of their Fourier components, for example f.sub.1 =13.56 up to f.sub.5 =67.8 MHz. This is conventionally achieved by high speed sampling using a sample frequency f.sub.s =&gt;f.sub.ny (f.sub.s equal to or greater than f.sub.ny), where f.sub.ny is the Nyquist frequency which is equal to 2.times.f.sub.a, where f.sub.a is the analogue bandwidth. In the above example f.sub.a =&gt;67.8 MHz. However, the signal-to-noise ratio (SNR) is limited by low bit resolution of the high speed flash converters currently available to operate at this high conversion speed f.sub.s. The equation for SNR is: EQU SNR=6.02n+1.6 dB+10log.sub.10 (f.sub.s /2f.sub.a)
where n is the number of bits of the converter, and a typical SNR of 50 dB is possible. This approach is expensive due to the requirement to use high speed RAM to store the converted values at the high sampling speed fs. An improvement is achieved by aliasing to reduce the required sample rate to below 13.56 MHz, as described in U.S. Pat. No. 5,565,737 using 12 bit converters. As the analogue bandwidth is still very high the SNR is maintained at approximately 50 dB but cheaper components can be employed and the dynamic range is increased by use of the 12 bit converters.
A problem with the known apparatus shown in FIG. 2 for obtaining V.sub.i and V.sub.v is that the accuracy of the current measurement by the loop 14 is reduced by the presence of stray magnetic flux originating from sources outside the RF conductor.
Problems with the known waveform sampling techniques described above are the low value of SNR which makes the calculation of the true plasma current and voltage difficult, and the use of non-coherent sampling which means that the record time does not contain an exact integer number of cycles. Time weighting of the samples is therefore required to reduce frequency side lobes on the Fourier transform and large samples sets are required to obtain reasonable values of phase resolution. Also, the frequency of sampling is determined by the fundamental frequency of the signal to be measured, and the system cannot handle the wide range of frequencies that are currently being used to bias chucks and develop new diagnostics and sensors (100 kHz to 27 MHz).